On July 2, 1864, Mosby learned from Jubal Early's quartermaster that Early was preparing to invade Maryland and march on Washington. Mosby decided to support the movement by cutting the telegraph line that ran along the Potomac from Washington to the Union garrison at Harpers Ferry. He called for a rendezvous at Rectortown the next morning; 250 of his Rangers responded. On July 4 they crossed the Potomac at Point of Rocks, Maryland, after a sharp fight with Federal infantry on Patton's Island in the middle of the river and the Loudoun Rangers in the town itself. The Rangers burned canal boats on the C&O Canal, cut the telegraph wires, and looted the warehouses. Some of the warehouses belonged to local Unionists, including the Loudoun Rangers' commander Samuel C. Means. The amount of fine clothing the Rangers carried back across the river gave the operation its name: the Calico Raid.

While Mosby was raiding Maryland, Colonel Charles Russell Lowell - Boston Brahmin, abolitionist, husband of Josephine Shaw Lowell - dispatched a hunting party to catch the Rangers. The force consisted of 100 troopers from the 2nd Massachusetts Cavalry and 50 from the 13th New York Cavalry, all under Major William H. Forbes. They left Falls Church on July 4 and moved west along the Little River Turnpike - now U.S. Route 50 - into Loudoun County. They reached Aldie and then Leesburg without finding any Rangers. On the morning of July 6, returning to Aldie along the Turnpike near Gum Springs, Forbes's pickets ran into Mosby's scouts. The Rangers had reorganized after the Calico Raid and were looking for exactly this kind of opportunity. The terrain was a low ridge with rolling fields and woodlots, with Mount Zion Church standing on a small rise just south of the road.

Mosby positioned his howitzer on the crest of the ridge with Sam Chapman to command it and formed his Rangers in columns of four on the Turnpike, with twelve skirmishers in advance under Lieutenant Harry Hatcher. Forbes assembled his men in two lines in a field south of the pike. Before he could charge, Chapman's howitzer fired - poorly aimed but disruptive. Forbes tried to redeploy. While he was reforming, Mosby's Rangers dismantled a rail fence that had stood between them and the Union line and then charged into the disorganized Federals. Their pistol volley sent the Union horses into panic. The Federal line broke and ran southwest past the Skinner house and Mount Zion Church. Forbes tried to rally a defensive line in the woods. The Federals drew their cavalry sabers; the Rangers fought with their .44 Colt revolvers. Sabers do not work well against pistol fire at close range. In the brawl in the trees, Forbes encountered Mosby and tried to cut him down. Richards took the blade. Mosby shot Forbes's horse and Forbes surrendered.

Among the Union dead was Captain Goodwin Stone of the 13th New York Cavalry, killed during the initial Ranger charge. The 12 killed, 37 wounded, and 57 captured - including Forbes himself - amounted to 71% of the original Federal force. Every Union horse that was not killed was taken back to Virginia as a Ranger remount. Mosby lost only one man dead and six wounded. The captured Union prisoners were marched south to Confederate prisoner exchange points; some of them spent the rest of the war in Richmond's Libby Prison. Forbes himself was exchanged after several months and survived the war. The Calico Raid's most lasting strategic consequence was indirect: the telegraph line Mosby had cut at Point of Rocks two days earlier was still down, and the destruction of communications between Washington and the Union garrisons in the Maryland panhandle slowed the Federal pursuit of Jubal Early after the Battle of Fort Stevens and the Confederate retreat from the city's outskirts.

Mount Zion Old School Baptist Church, the small brick chapel at the center of the action, was built in 1851. It still stands at the intersection of Route 50 and Mount Zion Road in Loudoun County, about a mile east of Aldie. The Northern Virginia Regional Park Authority now maintains the building, which is on the National Register of Historic Places. A small cemetery adjacent to the church holds graves from the 1850s onward. The Mosby Heritage Area Association has placed interpretive markers at the site, including one at the location where Ranger Richards took the saber meant for Mosby. From the air the site reads as a small rural intersection in the rolling horse country of central Loudoun County, with the brick church visible against the surrounding fields. The Bull Run Mountains rise about ten miles to the west. The Blue Ridge rises about twenty miles farther. The country looks now much as it looked in 1864 - except for the modern Route 50 traffic flowing through what was once the Little River Turnpike.